Energy conversion architecture with secondary side control delivered across transformer element

ABSTRACT

A switched mode power converter is configured having predominate secondary side control. A primary side driving circuit is configured as a responsive state machine the output of which is input as the driving signal for a main switch. An output voltage, current or power is sensed and the secondary side controller compares the sensed output characteristic with a predefined reference. The comparison results in an error that signifies an amount that the output is out of regulation. The secondary side controller drives a secondary side switch to generate a voltage pulse across the secondary winding. The voltage pulse has a pulse width that represents the amount of error in the output characteristic. The voltage pulse is transmitted across the transformer and received by the primary side driving circuit, which generates a driving signal modulated according to the voltage pulse and drives the main switch to regulate the output characteristic.

RELATED APPLICATIONS

This Patent Application claims priority under 35 U.S.C. 119 (e) of theU.S. Provisional Application Ser. No. 61/683,643, filed Aug. 15, 2012,and entitled “Reconstruction Pulse Shape Integrity in FeedbackEnvironment and the Bidirectional Energy Conversion Architecture” andthe U.S. Provisional Application Ser. No. 61/793,099, filed Mar. 15,2013, and entitled “New Power Management Integrated CircuitPartitioning”. This application incorporates U.S. ProvisionalApplication Ser. No. 61/683,643 and U.S. Provisional Application Ser.No. 61/793,099 in their entireties by reference.

FIELD OF THE INVENTION

The present invention is generally directed to the field of powerconverters. More specifically, the present invention is directed tocontrolling a power converter.

BACKGROUND OF THE INVENTION

In many applications a power converter is required to provide a voltagewithin a predetermined range formed from a voltage source having adifferent voltage level. Some circuits are subject to uncertain andundesirable functioning and even irreparable damage if supplied powerfalls outside a certain range. More specifically, in some applications,a precise amount of power is required at known times. This is referredto as regulated power supply.

In order to control a power converter to deliver a precise amount ofpower as conditions require, some form of control of the power converteris required. This control can occur on the primary side of an isolationtransformer or the secondary side. A closed loop feedback control systemis a system that monitors some element in the circuit, such as thecircuit output voltage, and its tendency to change, and regulates thatelement at a substantially constant value. Control on the secondary sideof a power converter can use a monitored output voltage as feedbackcontrol, but requires the use of some communication from the secondaryto the primary side of the isolation transformer to control the primaryside switching element. Control on the primary side can readily controlthe primary side switching element, but requires some feedback mechanismfrom the secondary side to the primary side to convey the status of themonitored element. In some applications, an optical coupler circuit, oropto coupler, is used to transmit feedback signals while maintainingelectrical isolation between the primary and secondary sides.

FIG. 1 illustrates a conventional regulated switch mode power converterincluding an optical coupler circuit. The power converter 2 isconfigured as a traditional flyback type converter. The power converter2 includes an isolation transformer 4 having a primary winding P1 and asecondary winding S1. The primary winding P1 is electrically coupled toan input voltage Vin and a driving circuit including a transistor 8, aresistor 12, and a controller 10. A capacitor 28 is coupled across theinput Vin and coupled with the primary winding P1. Input voltage to thecircuit may be unregulated DC voltage derived from an AC supply afterrectification and filtering. The transistor 8 is a fast-switchingdevice, such as a MOSFET, the switching of which is controlled by thefast dynamic controller 10 to maintain a desired output voltage Vout.The controller 10 is coupled to the gate of the transistor 8. As is wellknown, the DC/DC conversion from the primary winding P1 to the secondarywinding S1 is determined by the duty cycle of the PWM switching signalprovided to the transistor 8. The secondary winding voltage is rectifiedand filtered using the diode 6 and the capacitor 22. A sensing circuitand a load 14 are coupled in parallel to the secondary winding S1 viathe diode 6. The sensing circuit includes resistor 16, resistor 18, anda secondary controller 20. A secondary controller 20 senses the outputvoltage Vout across the load.

In this configuration, the power converter is controlled by drivingcircuitry on the primary side, and the load coupled to the output isisolated from the control. As such, a monitored output voltage used forvoltage regulation is required as feedback from the secondary side tothe control on the primary side. The power converter 2 has a voltageregulating circuit that includes the secondary controller 20 and anoptical coupler circuit. The optical coupled circuit includes twogalvanically isolated components, an optical diode 24 coupled to thesecondary controller 20 and an optical transistor 26 coupled to thecontroller 10. The optical diode 24 provides optical communication withthe optical transistor 26 across the isolation barrier formed by thetransformer 4. The optical coupler circuit in cooperation with thesecondary controller 20 provides feedback to the controller 10. Thecontroller 10 accordingly adjusts the duty cycle of the transistor 8 tocompensate for any variances in an output voltage Vout.

However, the use of an optical coupler circuit in and of itself presentsissues. Firstly, the optical coupler circuit adds extra cost. In someapplications, the optical coupler circuit can add more cost to the powerconverter than the isolation transformer. The optical coupler circuitalso adds to manufacturing and testing costs. Furthermore, theperformance of the optical coupler circuit degrades over time andtherefore introduces another potential point of failure in the overallpower converter. Also, characteristics of the optical coupler circuitmust be accounted for in the overall circuit design. For example, theoptical diode component is non-linear and as such a correlation betweenthe optical diode and the optical transistor must be established. Theoptical coupler circuit also has delays related to the operation of theoptical diode and the optical transistor, and the operation of theoptical diode requires a well defined DC level. As a result, it isgenerally desirable to avoid the use of an optical coupler circuit.

A next generation of feedback control does not use optical controlcircuitry. Instead, the transformer is used to convey real-time feedbacksignaling from the secondary side to the primary side. In such anapplication, the transformer includes an auxiliary winding on theprimary side that is magnetically coupled to the secondary winding. FIG.2 illustrates a conventional regulated power converter including amagnetically coupled feedback circuit. The power converter 32 isconfigured as a traditional flyback type converter. The power converter32 includes an isolation transformer 34 having a primary winding P1 anda secondary winding S1. The primary winding P1 is electrically coupledto an input voltage Vin and a driving circuit including a transistor 44,a resistor 46, and a controller 42. A capacitor 58 is coupled across theinput Vin and coupled with the primary winding P1. Input voltage to thecircuit may be unregulated DC voltage derived from an AC supply afterrectification and filtering. Similar to the power converter in FIG. 1,the transistor 44 is a fast-switching device controlled by the fastdynamic controller 42 to maintain a desired output voltage Vout. Thesecondary winding voltage is rectified and filtered using the diode 36and the capacitor 38, with the output voltage Vout delivered to the load40.

The power converter 32 has a feedback loop that includes a magneticallycoupled feedback circuit coupled to the secondary winding S1 of thetransformer 34 and the controller 42. The magnetically coupled feedbackcircuit includes a diode 48, a capacitor 50, resistors 52 and 54 and anauxiliary winding 56. The auxiliary winding 56 is coupled in parallel tothe series of resistors 52 and 54.

The voltage VA is proportional to the voltage across the auxiliarywinding 56. The voltage VA is provided as a feedback voltage VFB to thecontroller 42. The current through the transistor 44 is also provided asfeedback current IFB to the controller 42. The controller 42 includes areal-time waveform analyzer that analyzes input feedback signals, suchas the feedback voltage VFB and the feedback current IFB.

The auxiliary winding 56 is also magnetically coupled to the secondarywinding S1. When the current through the diode 36 is zero, the voltageacross the secondary winding S1 is equal to the voltage across theauxiliary winding 56. This relationship provides means for communicatingthe output voltage Vout as feedback to the primary side of the circuit.The voltage across the auxiliary winding 56 is measured when it isdetermined that the current through the diode 36 is zero, which providesa measure of the voltage across the secondary winding S1 and thereforethe output voltage Vout.

The feedback voltage VFB when the diode 36 current is zero is determinedand is referred to as the “voltage sense”, and the feedback current IFBwhen the diode 36 current is zero is determined and is referred to asthe “current sense”. The voltage sense and the current sense are inputto the real-time waveform analyzer within the controller 42. FIG. 3illustrates a functional block diagram of a conventional real-timewaveform analyzer 60. Error amplifiers 62 and 64 are acceptors of theregulating means, which in this configuration are the sensed voltage andthe sensed current. The error amplifier compares the input sensedvoltage to a reference voltage and outputs a first difference value. Thefirst difference value is amplified by the gain of the error amplifier62. The error amplifier 64 compares the amplified first difference valueto the sensed current and outputs a second difference value that iseither High or Low. A pulse width modulation (PWM) block 66 isconfigured as a Flip-Flop digital device. The output of the PWM block 66is set according to the switching frequency of the clock 68 and is Resetby the High or Low value input from the error amplifier 64. The variablesignal applied to the Reset pin generates an output signal that is apulse train modulated by the pulse width. A multiple input OR gate 70inputs the clock signal, the pulse train signal, a shutdown signal, anda OVP/UVP/OTP signal, where OVP stands for “over voltage protection”,UVP stands for “undervoltage protection” and OTP stands for “overtemperature protection”. The waveform analyzer is configured to output ahigh voltage value when one of the inputs to the OR gate is high or tooutput a low voltage value when all of the inputs to the OR gate arelow. The high voltage value output from the waveform analyzercorresponds to turning on the transistor 44 in FIG. 2. The low voltagevalue corresponds to turning off the transistor 44. The OR gate alsoenables a high voltage signal output from the PWM block 66 to propagateto the output by monitoring abnormal conditions such as under voltage,over voltage, over temperature, etc. In this manner, the pulse width ofeach pulse can be modified which adjusts the output voltage intoregulation.

In general, control intricacies of the waveform analyzer are alignedwith control argument sampling to achieve overall system functionalperformance. Sampling argument is in the form of current, voltage andimpedance. System functional performance is in the form of pulse widthmodulation (PWM), pulse frequency modulation (PFM) and pulse amplitudemodulation (PAM). The waveform analyzer of FIG. 3 is limited to signalprocessing in DC or real-time switching waveforms. In either case, thefeedback signal received by the waveform analyzer requires some statusintegrity, such as no noise on the DC level, no disturbance on theswitching waveform and to some degree represent a combination of analogand digital representations. The voltage across the auxiliary windingtypically forms a pulse train with frequency corresponding to theswitching frequency of the driving transistor. The voltage across theauxiliary winding when the secondary winding current is zero, whichcorresponds to the diode 36 current equaling zero, corresponds to thefalling edge of the pulse. As such, measuring an accurate voltage valuerequires that the pulse is well defined with sufficient pulse integrityparticularly at the falling edge. Further, the voltage value immediatelyfollowing the rising edge includes ringing due to the leakage impedanceof the transformer. As such, pulse integrity also requires sufficienttime for the voltage value to stabilize following the rising edge.Higher switching frequencies minimize the pulse width and thereforeprovide less time for voltage stabilization. For at least these reasons,providing a pulse with sufficient pulse integrity is often difficult toachieve.

SUMMARY OF THE INVENTION

A switched mode power converter is configured having predominatesecondary side control. In some embodiments, a primary side drivingcircuit is configured as a responsive state machine, such as a pulsewidth modulating (PWM) circuit, the output of which is input as thedriving signal for a main switch. One or more output characteristics,such as output voltage, current or power, are sensed, and the secondaryside controller compares the sensed output characteristic(s) with apredefined reference. The comparison results in a difference, or anerror, that signifies an amount that the output is out of regulation.The secondary side controller drives a secondary side switch so as togenerate a negative secondary current through a secondary winding, whichresults in a voltage pulse across the secondary winding. The secondaryside switch is driven so as to form a voltage pulse having a pulse widththat represents the amount of error in the output characteristic. Thepulse width fluctuates according to the error value. The voltage pulseis transmitted across the transformer and received by the primary sidedriving circuit, which generates a driving signal modulated according tothe voltage pulse and drives the main switch to regulate the outputcharacteristic.

In an aspect, a method of controlling a switching mode power converteris disclosed. The method includes configuring a switching mode powerconverter. The power converter includes a transformer, an output circuitcoupled to a secondary winding of the transformer, a first switchcoupled to a primary winding of the transformer and a driving circuitcoupled to the first switch. The output circuit includes a second switchcoupled to the secondary winding of the transformer and a controllercoupled to the second switch. The method also includes measuring anoutput characteristic of the output circuit. The method also includesgenerating a driving signal by the controller and applying the drivingsignal to the second switch. The driving signal is modulated based onthe measured output characteristic, and applying the driving signal tothe second switch results in a voltage pulse across the secondarywinding. The voltage pulse includes a pulse width that corresponds to amagnitude of an out of regulation output characteristic. The method alsoincludes transmitting the voltage pulse through the transformer to thedriving circuit and modulating a driving signal generated by the drivingcircuit according to the voltage pulse. The method also includes drivingthe first switch using the driving signal to regulate the outputcharacteristic.

The method can also include determining for each switching cycle of thepower converter when a power delivery from a primary side of thetransformer to the output circuit is completed, and when the powerdelivery is completed then generating the driving signal andtransmitting the voltage pulse. The power delivery can be completed whena current through the secondary winding drops to zero. The powerdelivery from the primary side to the output circuit can correspond tothe first switch being OFF, and generating the driving signal andtransmitting the voltage pulse can be performed while the first switchis OFF. The driving circuit can include a pulse width modulation circuitand the driving signal can be a pulse width modulated voltage signal.The voltage pulse received by the pulse width modulation circuit canmodulate a duty cycle of the pulse width modulated voltage signal outputfrom the pulse width modulation circuit. The output characteristic canbe one or more of an output voltage, an output current, and an outputpower of the power converter. The voltage pulse can be formed across thesecondary winding while the second switch is ON which enables a negativesecondary current through the secondary winding. Enabling the negativesecondary current can include enabling an alternative current path froman output capacitor in the output circuit to the secondary winding. Thesecondary winding can be magnetically coupled to an auxiliary winding,and the auxiliary winding can be coupled to the driving circuit, furtherwherein transmitting the voltage pulse can include using the magneticcoupling between the secondary winding and the auxiliary winding totransmit the voltage pulse from the secondary winding to the drivingcircuit. Transmitting the voltage pulse can include using a parasiticcapacitance between a primary side and a secondary side of the powerconverter. The parasitic capacitance can be a parasitic capacitance ofthe transformer. The parasitic capacitance can be an inherentcapacitance of a printed circuit board onto which the power converter isassembled, wherein the inherent capacitance is a result of a componentlayout on either side of an isolation galvanic barrier. The first switchcan be a first transistor and the second switch can be a secondtransistor.

In another aspect, a switching mode power converter is disclosed. Thepower converter includes a transformer, a first switch, a drivingcircuit, a second switch, a controller, and a sensing circuit. Thetransformer has a primary winding coupled to an input supply voltage anda secondary winding. The first switch is coupled in series to theprimary winding. The driving circuit is coupled to the switch, whereinthe driving circuit is configured to drive the first switch ON and OFF.The second switch is coupled in series to the secondary winding. Thecontroller is coupled to the second switch, wherein the secondcontroller is configured to turn the second switch ON and OFF. Thesensing circuit is coupled to the secondary winding and the controller,wherein the sensing circuit is configured to sense an outputcharacteristic of the power converter. The controller is configured togenerate a second switch driving signal and apply the second switchdriving signal to the second switch. The second switch driving signal ismodulated based on the measured output characteristic, and applying thesecond switch driving signal to the second switch results in a voltagepulse across the secondary winding. The voltage pulse includes a pulsewidth that corresponds to a magnitude of an out of regulation outputcharacteristic. The transformer is configured as a signal transmitter totransmit the voltage pulse from the secondary winding to a primary sideof the transformer. The driving circuit is configured to generate afirst switch driving signal modulated according to the voltage pulse andto drive the first switch using the first switch driving signal toregulate the output characteristic.

The controller and the sensing circuit can be configured to determinefor each switching cycle of the power converter when a power deliveryfrom a primary side of the transformer to the output circuit iscompleted, and when the power delivery is completed then the controlleris configured to generate the second switch driving signal. The powerdelivery can be completed when a current through the secondary windingdrops to zero. The power delivery from the primary side to the outputcircuit can correspond to the first switch being OFF, and the controllercan be configured to generate the driving signal while the first switchis OFF. The driving circuit can be a pulse width modulation circuit andthe driving signal can be a pulse width modulated voltage signal. Thepulse width modulation circuit can be configured to modulate a dutycycle of the pulse width modulated voltage signal output from the pulsewidth modulation circuit based on the received voltage pulse. The outputcharacteristic can be one or more of an output voltage, an outputcurrent, and an output power of the power converter. The power convertercan also include an auxiliary winding coupled to the driving circuit,wherein the auxiliary winding is magnetically coupled to the secondarywinding. The transformer can be configured to transmit the voltage pulsefrom the secondary winding to the auxiliary winding using the magneticcoupling between the secondary winding and the auxiliary winding. Thetransformer can be configured to transmit the voltage pulse from thesecondary winding to the auxiliary winding using a parasitic capacitancebetween a primary side and a secondary side of the power converter. Theparasitic capacitance can be a parasitic capacitance of the transformer.The parasitic capacitance can be an inherent capacitance of a printedcircuit board onto which the power converter is assembled, wherein theinherent capacitance is a result of a component layout on either side ofan isolation galvanic barrier. The first switch can be a firsttransistor and the second switch can be a second transistor. The sensingcircuit can include a voltage divider circuit. The power converter canalso include a diode coupled in parallel to the second switch and anoutput capacitor coupled in series to the diode, wherein the diode isconfigured to enable current flow from the secondary winding to theoutput capacitor when forward-biased. When the second switch is ON, analternative current path can be formed between the output capacitor andthe secondary winding of the transformer, further wherein a negativesecondary current flows from the output capacitor to the secondarywinding via the alternative current path, thereby forming the voltagepulse across the secondary winding. The negative secondary current canbe generated as discharge from the output capacitor when the alternativecurrent path is formed. The power converter can be configured as one ofa flyback-type power converter circuit, a forward-type power convertercircuit, a push-pull-type power converter circuit, a half-bridge-typepower converter circuit, and a full-bridge-type power converter circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Several example embodiments are described with reference to thedrawings, wherein like components are provided with like referencenumerals. The example embodiments are intended to illustrate, but not tolimit, the invention. The drawings include the following figures:

FIG. 1 illustrates a conventional regulated switch mode power converterincluding an optical coupler circuit.

FIG. 2 illustrates a conventional regulated power converter including amagnetically coupled feedback circuit.

FIG. 3 illustrates a functional block diagram of a conventionalreal-time waveform analyzer.

FIG. 4 illustrates a power converter according to an embodiment.

FIG. 5 illustrates a functional block diagram of a portion of thecontroller for processing the coded voltage pulse train according to anembodiment.

FIG. 6 illustrates a power converter according to another embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present application are directed to a powerconverter. Those of ordinary skill in the art will realize that thefollowing detailed description of the power converter is illustrativeonly and is not intended to be in any way limiting. Other embodiments ofthe power converter will readily suggest themselves to such skilledpersons having the benefit of this disclosure.

Reference will now be made in detail to implementations of the powerconverter as illustrated in the accompanying drawings. The samereference indicators will be used throughout the drawings and thefollowing detailed description to refer to the same or like parts. Inthe interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application and business related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

FIG. 4 illustrates a power converter according to an embodiment. Thepower converter 100 is configured to receive an unregulated DC voltagesignal at an input node Vin and to provide a regulated output voltageVout. Input voltage to the circuit may be unregulated DC voltage derivedfrom an AC supply after rectification. The input voltage is typicallyfiltered, such as via capacitor 102.

The power converter 100 is configured as a flyback converter. It isunderstood that the concepts described herein can be applied toalternatively configured switched mode converters including, but notlimed to, a forward converter, a push-pull converter, a half-bridgeconverter, and a full-bridge converter. The power converter 100 includesan isolation transformer 104 having a primary winding P1 and a secondarywinding S1. The primary winding P1 is electrically coupled to the inputvoltage Vin and a driving circuit including a switch 106, a senseresistor 112, and a controller 110. The switch 106 is coupled in serieswith the primary winding P1 of the transformer 104 and the senseresistor 112. The controller 110 is coupled to the switch 106 to turnthe switch ON and OFF.

The power converter 100 further includes output circuitry coupled to thesecondary winding S1 of the transformer 104. The output circuitryincludes a freewheeling rectifier diode 116, a switch 118, a controller120, and an output capacitor 126. The switch 118 is coupled in parallelto the diode 116. An anode of the diode 116 is coupled to a firstterminal of the secondary winding. A cathode of the diode 116 is coupledto a first terminal of the output capacitor 126 and coupled to theoutput node Vout. The output capacitor 126 is coupled to the Vout nodeacross an output load, represented by a resistor 128. The controller 120is coupled to the switch 118 to turn the switch ON and OFF. The outputcircuitry also includes a sensing circuit configured to measure acircuit characteristic to be regulated such as an output voltage, anoutput current, and/or an output power. In this exemplary configurationand succeeding description, the power circuit is described as sensingand regulating the output voltage Vout. In the exemplary configurationof FIG. 2, the sensing circuit includes a resistive voltage dividerincluding the resistors 122 and 124 coupled in parallel to the capacitor126 to measure a voltage across the capacitor 126. It is understood thatan alternative sensing circuit can be used to measure the output voltageVout. In general, the sensing circuit can be configured to use anyconventional technique for determining the value of the regulatedcircuit characteristic.

The switch 106 and the switch 118 are each a suitable switching device.In an exemplary embodiment, the switch 106 and the auxiliary switch 118are each a n-type metal-oxide-semiconductor field-effect transistor(MOSFET) device. Alternatively, any other semiconductor switching deviceknown to a person of skill in the art can be substituted for the switch106 and/or the switch 118. Subsequent description is based on ann-channel MOSFET.

The power converter 100 has a feedback loop that includes a magneticallycoupled feedback circuit coupled to the secondary winding S1 of thetransformer 104 and the controller 110. The magnetically coupledfeedback circuit includes a diode 108, a capacitor 130, resistors 132and 134 and an auxiliary winding 114. The auxiliary winding 114 iscoupled in parallel to the series of resistors 132 and 134. Theauxiliary winding 114 is also magnetically coupled to the secondarywinding S1. When the current through the diode 116 is zero, the voltageacross the secondary winding S1 is equal to the voltage across theauxiliary winding 114 if the turns ratio is 1:1, or otherwiseproportional depending on the turns ratio. This relationship providesmeans for communicating the voltage across the secondary winding S1 asfeedback to the primary side of the circuit. The value of the voltageacross the secondary winding S1 is a function of the secondary currentthrough the secondary winding S1. With the current through the diode 116equal to zero, the transistor 118 is selectively turned ON and OFF bythe controller 120. When the transistor 118 is ON, an alternativecurrent path is formed from the charged capacitor 126 to the secondarywinding S1. The alternative current path enables negative current flowthrough the secondary winding S1. In this manner, the controller 120generates a driving signal that selectively turns the transistor 1180Nand OFF, thereby generating a coded train of voltage pulses across thesecondary winding S1. The driving signal is configured such that thevoltage pulses are modulated with coded information. In this manner, acoded voltage pulse train is transmitted during a delay period thatcorresponds to the switch 106 OFF and the secondary winding currenthaving dropped to zero.

In some embodiments, the coded information is the measured outputcircuit characteristic that is to be regulated, such as the outputvoltage Vout. In this case, the controller 120 receives the sensedoutput voltage Vout, and generates a driving signal resulting in amodulated train of voltage pulses across the secondary winding S1 thatis coded to conveys the sensed output voltage Vout. In this manner, acoded signal is generated in the form of a coded voltage pulse train,where the DC level of the measured output voltage Vout is coded into thecoded signal. Coded information is included in the coded pulse train bymodulating pulses of the pulse train including, but no limited to, thepulse width, the pulse amplitude, the pulse frequency, or anycombination thereof. For example, the pulse train can be modulated bythe number of pulses over a predetermined time period, or the number ofpulses with different amplitudes over the time period.

The auxiliary winding 114 is magnetically coupled to the secondarywinding S1, and the voltage across the auxiliary winding 114 is equal toor proportional to the voltage across the secondary winding S1 when thecurrent through the diode 116 is zero. As such, the coded voltage pulsetrain is transmitted from across the isolation galvanic barrier via themagnetically coupled auxiliary winding 114 and secondary winding S1.

The coded voltage pulse train across the auxiliary winding 114 ismeasured when the transistor 106 is OFF and the current through thediode 116 equals zero. The voltage VA is proportional to the voltageacross the auxiliary winding 114 and therefore represents the codedvoltage pulse train. The voltage VA is provided as a feedback voltageVFB to the controller 110, wherein the feedback voltage VFB representsthe coded voltage pulse train. In contrast to the conventional powerconverter of FIG. 2 where the feedback voltage VFB is a single pulse perswitching cycle of the main transistor 44, the feedback voltage VFBinput to the controller 110 is a train of pulses per switching cycle ofthe main transistor 106. The train of pulses includes the codedinformation that identifies the measured output voltage Vout, again incontrast to the conventional power converter of FIG. 2 where the singlepulse represents the actual output voltage Vout.

The controller 110 is configured to receive the feedback voltage FB. Thecurrent through the transistor 106 is also provided as feedback currentIFB to the controller 110. The controller 110 includes a real-timewaveform analyzer that analyzes input feedback signals, such as thefeedback voltage VFB and the feedback current IFB. FIG. 5 illustrates afunctional block diagram of a portion of the controller 110 forprocessing the coded voltage pulse train according to an embodiment. Thefeedback voltage VFB input to the controller 110 is received by a pulsetrain acceptor 140. The pulse train acceptor is a digital filterelement, such as a high pass filter, that filters the received codedvoltage pulse train. The filtered signal output from the pulse trainacceptor 140 is input to a signal integrity discriminator 142 where thesignal is decoded and the measured output voltage Vout coded into thecoded voltage pulse train is identified. The signal integritydiscriminator 142 generates and outputs a driving signal thatcorresponds to the identified output voltage Vout. The driving signal isinput to a digital to analog converter 144 and converted to acorresponding DC level.

The DC level output from the converter 144 is input to the waveformanalyzer 146 as the “voltage sense”. The feedback current IFB is inputto the waveform analyzer as the “current sense”. The voltage sense isprovided as a first input to an error amplifier 148. The current senseis provided as a first input to the error amplifier 150. Erroramplifiers 148 and 150 are acceptors of the regulating means, which inthis configuration are the voltage sense and the current sense. Theerror amplifier 148 compares the input voltage sense to a referencevoltage and outputs a first difference value. The first difference valueis amplified by the gain of the error amplifier 148. The error amplifier150 compares the amplified first difference value to the current senseand outputs a second difference value that is either High or Low. Apulse width modulation (PWM) block 152 is configured as a Flip-Flopdigital device. The output of the PWM block 152 is set according to theswitching frequency of the clock 154 and is Reset by the High or Lowvalue input from the error amplifier 150. The variable signal applied tothe Reset pin generates an output signal that is a pulse train modulatedby the pulse width. A multiple input OR gate 156 inputs the clocksignal, the pulse train signal, a shutdown signal, and a OVP/UVP/OTPsignal. The OR gate 156 outputs a high voltage value when one of theinputs to the OR gate is high or to output a low voltage value when allof the inputs to the OR gate are low. The output of the OR gate 156 isamplified by amplifier 158 and output to drive the gate of thetransistor 106 (FIG. 4). The high voltage value output from the OR gate156 corresponds to turning ON the transistor 106 in FIG. 4. The lowvoltage value output from the OR gate 156 corresponds to turning OFF thetransistor 106. The OR gate 156 also enables a high voltage value topropagate to the output by monitoring abnormal conditions such as undervoltage, over voltage, over temperature, etc. In this manner, the pulsewidth of each pulse output from the PWM block 152 can be modified toadjust the output voltage into regulation.

In general, control intricacies of the waveform analyzer are alignedwith control argument sampling to achieve overall system functionalperformance. Sampling argument is in the form of current, voltage andimpedance. System functional performance is in the form of pulse widthmodulation (PWM), pulse frequency modulation (PFM) and pulse amplitudemodulation (PAM).

In operation, a circuit output characteristic is measured on thesecondary side of a switching mode power converter. In an exemplaryapplication, the circuit output characteristic is the output voltageVout. The secondary side controller generates a driving signal forcontrolling the secondary transistor while the primary side maintransistor is OFF. The driving signal selectively turns ON and OFF thesecondary transistor resulting in a coded train of voltage pulses acrossthe secondary winding. The measured output voltage Vout is coded intothe coded voltage pulse train. In some embodiments, the coded voltagepulse train is transmitted from the secondary winding to the auxiliarywinding by magnetic coupling between the two windings. In otherembodiments, the coded voltage pulse train is transmitted form thesecondary winding to the auxiliary winding using the parasiticcapacitance of either the transformer or the inherent capacitance of theprinted circuit board across the isolation galvanic barrier, where theprinted circuit board capacitance is due to the formation of thetransformer and corresponding circuitry layout of the power convertercomponents. The coded voltage pulse train is received and decoded by theprimary controller. The primary controller identifies the measuredoutput voltage Vout according to the decoded information and generates adriving signal corresponding to the identified output voltage Vout. Thedriving signal is converted to a DC level that is input as the voltagesense to a waveform analyzer. The waveform analyzer uses the inputvoltage sense to generate a driving signal for controlling the primarytransistor and regulating the output voltage Vout.

In an alternative configuration, a bi-directional switch is used inplace of the diode 116 and the transistor 118. A body diode of thebi-directional switch functions as the freewheeling diode 116. Controlof the bi-directional switch is the same as the transistor 118 to enablea negative secondary current Isec.

Although the coded information within the coded voltage pulse train isdescribed above as including information that identifies the outputvoltage Vout, the coded voltage pulse train can be alternativelymodulated to include additional or alternative information. Suchinformation includes, but is not limited to, a simple instruction toturn ON or OFF the main transistor, an indicator of a short circuitcondition, or an indicator of a voltage out of regulation. Each type ofinformation is represented by a separate coding.

In the embodiment described above, the predominate control functionalityis performed by the primary side controller. In some embodiments, thepredominate control functionality is instead performed by the secondside controller. This configuration is particularly useful in thoseapplications where the primary side voltages are high, such as 100-400V,and the secondary side voltages are lower, such as 20-40V. Circuits thatprocess lower voltage signals are easier to implement as integratedcircuits. Integration at higher voltages is extremely complex.

A power converter having predominate secondary side controlcorrespondingly reduces the intelligent control on the primary side.Essentially, the primary side controller is configured as a responsivestate machine that responds to signaling provided by the secondary sidecontroller. In some embodiments, the primary side controller of thepower converter in FIG. 4 is replaced by a modulating or drivingcircuit, such as a pulse width modulating (PWM) circuit, the output ofwhich is input as the driving signal for the main switch. Thefundamental functionality of the power converter having predominatesecondary side control is the same as the power converter of FIG. 4where processing is re-portioned from the primary side to the secondaryside.

FIG. 6 illustrates a power converter according to an embodiment. Thepower converter 200 is configured to receive an unregulated DC voltagesignal at an input node Vin and to provide a regulated output voltageVout. Input voltage to the circuit may be unregulated DC voltage derivedfrom an AC supply after rectification. The input voltage is typicallyfiltered, such as via capacitor 202.

The power converter 200 is configured as a flyback converter. It isunderstood that the concepts described herein can be applied toalternatively configured switched mode converters including, but notlimed to, a forward converter, a push-pull converter, a half-bridgeconverter, and a full-bridge converter. The power converter 200 includesan isolation transformer 204 having a primary winding P1 and a secondarywinding S1. The primary winding P1 is electrically coupled to the inputvoltage Vin and a switch 206. A driving circuit includes a pulse widthmodulating (PWM) circuit 210. The switch 206 is coupled in series withthe primary winding P1 of the transformer 204 and a resistor 212. ThePWM circuit 210 is coupled to the switch 206 to turn the switch ON andOFF.

The power converter 200 further includes output circuitry coupled to thesecondary winding S1 of the transformer 204. The output circuitryincludes a freewheeling rectifier diode 216, a switch 218, a controller220, and an output capacitor 226. The switch 218 is coupled in parallelto the diode 216. An anode of the diode 216 is coupled to a firstterminal of the secondary winding. A cathode of the diode 216 is coupledto a first terminal of the output capacitor 226 and coupled to theoutput node Vout. The output capacitor 226 is coupled to the Vout nodeacross an output load, represented by a resistor 228. The controller 220is coupled to the switch 218 to turn the switch ON and OFF. The outputcircuitry also includes a sensing circuit configured to measure acircuit characteristic to be regulated such as an output voltage, anoutput current, and/or an output power. In this exemplary configurationand succeeding description, the power converter is described as sensingand regulating the output voltage Vout. In the exemplary configurationof FIG. 4, the sensing circuit includes a resistive voltage dividerincluding the resistors 222 and 224 coupled in parallel to the capacitor226 to measure a voltage across the capacitor 226. It is understood thatan alternative sensing circuit can be used to measure the output voltageVout. In general, the sensing circuit can be configured to use anyconventional technique for determining the value of the regulatedcircuit characteristic.

The output characteristic, such as the voltage, current or power, issensed, and the controller 220 compares the sensed outputcharacteristic(s) with a predefined reference. The comparison results ina difference, or an error, that signifies an amount that the output isout of regulation. The controller 220 drives the switch 206 so as togenerate a negative secondary current through the secondary winding S1,which results in a voltage pulse across the secondary winding 51. Theswitch 206 is driven so as to form a voltage pulse having a pulse widththat represents the amount of error in the output characteristic.

The switch 206 and the switch 218 are each a suitable switching device.In an exemplary embodiment, the switch 206 and the switch 218 are each an-type metal-oxide-semiconductor field-effect transistor (MOSFET)device. Alternatively, any other semiconductor switching device known toa person of skill in the art can be substituted for the switch 206and/or the switch 218. Subsequent description is based on an n-channelMOSFET.

The power converter 200 has a feedback loop that includes a magneticallycoupled feedback circuit coupled to the secondary winding S1 of thetransformer 204 and the PWM circuit 210. The magnetically coupledfeedback circuit includes a diode 208, a capacitor 230, resistors 232and 234 and an auxiliary winding 214. The auxiliary winding 214 iscoupled in parallel to the series of resistors 232 and 234. Theauxiliary winding 214 is also magnetically coupled to the secondarywinding S1. When the current through the diode 216 is zero, the voltageacross the secondary winding S1 is equal to the voltage across theauxiliary winding 114 if the turns ratio is 1:1, or otherwiseproportional depending on the turns ratio. This relationship providesmeans for communicating the voltage pulse across the secondary windingS1 as a control signal to the primary side of the circuit. The value ofthe voltage across the secondary winding S1 is a function of thesecondary current through the secondary winding S1. With the currentthrough the diode 216 equal to zero, the transistor 218 is selectivelyturned ON and OFF by the controller 220. When the transistor 218 is ON,an alternative current path is formed from the charged capacitor 226 tothe secondary winding S1. The alternative current path enables negativecurrent flow through the secondary winding S1. The negative secondarycurrent is sustained by the stored energy in the capacitor 226. In thismanner, the controller 220 generates a driving signal that selectivelyturns the transistor 218 ON and OFF, thereby generating a voltage pulsesignal across the secondary winding S1. The driving signal is configuredsuch that the pulse width of the voltage pulse is modulated according tothe compared difference between the measured output characteristic and areference value. The voltage pulse signal is transmitted during a delayperiod that corresponds to the switch 206 OFF and the current of thediode 216 having dropped to zero. The delay period is implemented by thecontroller 220.

The auxiliary winding 214 is magnetically coupled to the secondarywinding S1, and the voltage across the auxiliary winding 214 is equal toor proportional to the voltage across the secondary winding S1 when thecurrent through the diode 216 is zero. As such, the voltage pulse signalis transmitted across the isolation galvanic barrier via themagnetically coupled auxiliary winding 214 and secondary winding S1.

The voltage pulse signal across the auxiliary winding 214 is measuredwhen the transistor 206 is OFF and the current through the diode 216equals zero. The voltage VA is proportional to the voltage across theauxiliary winding 214 and therefore represents the voltage pulse signal.The voltage VA is provided as an input to the PWM circuit 210, where thevoltage VA represents the voltage pulse signal.

In operation, the output voltage, current or power is sensed, and thecontroller 220 compares the sensed output characteristic(s) with apredefined reference. The comparison results in a difference, or anerror, that signifies an amount that the output is out of regulation.The controller 220 drives the switch 218 so as to generate a negativesecondary current through the secondary winding S1, which results in avoltage pulse across the secondary winding S1. The switch 218 is drivenso as to form a voltage pulse having a pulse width that represents theamount of error in the output characteristic. The pulse width fluctuatesaccording to the error value. The negative secondary current is drivenduring a predetermined time cycle of the power converter main switchingelement 206 on the primary side. This predetermined time cyclecorresponds to the period of the main switching cycle when the mainswitching element 206 is OFF and after the power has been delivered tothe load, which corresponds to the positive secondary current droppingto zero. Also during the predetermined time cycle, the voltage pulse istransmitted across the galvanic isolation barrier and received by thedriving circuit on the primary side. The PWM circuit 210 generates adriving signal modulated according to the voltage pulse and drives themain switch 206 to regulate the output characteristic. The main operateswith a duty cycle controlled by the secondary side controller 220 by wayof the voltage pulse. In the absence of the voltage pulse transmittedthrough the transformer, the PWM block generates a pulse width modulateddriving signal having a predetermined duty cycle set by an input clocksignal. The voltage pulse functions to adjust or maintain thispredetermined duty cycle. Additionally, the voltage pulse can be usedfor functionality other than regulating means. For example, the voltagepulse can be used to turn the main switch ON or OFF immediately due tomonitored circuit conditions including, but not limited to, over voltagecondition, under voltage condition, and over temperature condition.

Conceptually, the power converter is configured for bi-directionalenergy conversion. In a first direction, there is a primary side tosecondary side energy conversion across the galvanic isolation barrier.Energy is transmitted from AC main through the primary of the powertransformer to the secondary. Energy at the secondary is rectified andpassed to the load as output power. In a second direction, energy istransmitted from the secondary side to the primary side as a controlsignal for modulating a duty cycle of the main switch and regulating theoutput. The energy transfer is the second direction is for signaling,data, and/or control means, not power transfer. To generate the propercontrol signal, the load condition is observed by a supervisorycircuitry integrated into a smart rectifier, measured against referencevalues to identify an error and prepare for transmission to the primaryside driving circuit for appropriate adjustment to compensate for lossof regulation. A secondary side controller, switch, and diode can beused to implement the smart rectifier. The supervised circuitry holdsthe error information and when energy transmission in the firstdirection is completed converts the error information into switchingmeans, such as in the form of a fluctuated pulse width signal. Thefluctuated pulse width signal activates the smart rectifier intonegative secondary current and transmits the sampled data through thetransformer. A driving circuit modulates the main switch driving signalaccording to the received pulse width signal. In some embodiments, thedriving circuit functions as a responsive state machine that inputs thepulse width signal and modulates the output main switch driving signalaccording to the input pulse width signal. In other embodiments, thedriving circuit includes a primary side controller that reconstructs thesignal, uses the pulse width to identify the error information, andmakes the decision to change timing characteristics of the main switch.

In the context of the bi-directional energy conversion, the transformercan be viewed from a master-slave standpoint. When the main switch is ONand the secondary side switch is OFF and energy is transferred as poweracross the transformer, as in the first direction of energy conversion,the primary side of the circuit functions as a master circuit to thetransformer. When the main switch is OFF and the secondary side switchis ON and energy is transferred as control means across the transformer,as in the second direction of energy conversion, the secondary side ofthe circuit functions as a master circuit to the transformer.

Although the energy conversions in the first and second directions arepreferred to be serial, it is also contemplated that the energyconversions are concurrent in both directions. In a concurrent energyconversion, the switching frequency of the secondary side switch isdistinctly higher than the switching frequency of the primary side mainswitch.

The driving circuit for the main switch is implemented above as a PWMcircuit. Alternatively, system functional performance can be in the formof pulse width modulation (PWM), pulse frequency modulation (PFM) andpulse amplitude modulation (PAM).

The present application has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the power converter. Many ofthe components shown and described in the various figures can beinterchanged to achieve the results necessary, and this descriptionshould be read to encompass such interchange as well. As such,references herein to specific embodiments and details thereof are notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications can be made tothe embodiments chosen for illustration without departing from thespirit and scope of the application.

What is claimed is:
 1. A method of controlling a switching mode powerconverter comprising: a. configuring a switching mode power convertercomprising a transformer, an output circuit coupled to a secondarywinding of the transformer, a first switch coupled to a primary windingof the transformer and a state machine coupled to the first switch,wherein the output circuit comprises a second switch coupled to thesecondary winding of the transformer and a controller coupled to thesecond switch; b. measuring an output characteristic of the outputcircuit; c. generating a second switch driving signal by the controllerand applying the second switch driving signal to the second switch,wherein the second switch driving signal is modulated based on themeasured output characteristic, and applying the second switch drivingsignal to the second switch results in a first voltage pulse across thesecondary winding, the first voltage pulse comprising a pulse width thatcorresponds to a magnitude of an out of regulation outputcharacteristic; d. transmitting the first voltage pulse through thetransformer and inputting a second voltage pulse proportional to thefirst voltage pulse across the secondary winding to the state machine;e. generating a first switch driving signal by the state machine,wherein the first switch driving signal is modulated according to thepulse width; and g. driving the first switch using the first switchdriving signal to regulate the output characteristic.
 2. The method ofclaim 1 further comprising determining for each switching cycle of thepower converter when a power delivery from a primary side of thetransformer to the output circuit is completed, and when the powerdelivery is completed then generating the driving signal andtransmitting the first voltage pulse.
 3. The method of claim 2 whereinthe power delivery is completed when a current through the secondarywinding drops to zero.
 4. The method of claim 2 wherein the powerdelivery from the primary side to the output circuit corresponds to thefirst switch being OFF, and generating the second switch driving signaland transmitting the first voltage pulse are performed while the firstswitch is OFF.
 5. The method of claim 1 wherein the state machine is apulse width modulation circuit and the first switch driving signalcomprises a pulse width modulated voltage signal.
 6. The method of claim5 wherein the second voltage pulse received by the pulse widthmodulation circuit modulates a duty cycle of the pulse width modulatedvoltage signal output from the pulse width modulation circuit.
 7. Themethod of claim 1 wherein the output characteristic is one or more of anoutput voltage, an output current, and an output power of the powerconverter.
 8. The method of claim 1 wherein the first voltage pulse isformed across the secondary winding while the second switch is ON whichenables a negative secondary current through the secondary winding. 9.The method of claim 8 wherein enabling the negative secondary currentcomprises enabling an alternative current path from an output capacitorin the output circuit to the secondary winding.
 10. The method of claim1 wherein the secondary winding is magnetically coupled to an auxiliarywinding, and the auxiliary winding is coupled to the state machine,further wherein transmitting the first voltage pulse comprises using themagnetic coupling between the secondary winding and the auxiliarywinding to transmit the first voltage pulse from the secondary windingthrough the transformer such that the second voltage pulse is input tothe state machine.
 11. The method of claim 1 wherein transmitting thefirst voltage pulse comprises using a parasitic capacitance between aprimary side and a secondary side of the power converter.
 12. The methodof claim 11 wherein the parasitic capacitance comprises a parasiticcapacitance of the transformer.
 13. The method of claim 11 wherein theparasitic capacitance comprises an inherent capacitance of a printedcircuit board onto which the power converter is assembled, wherein theinherent capacitance is a result of a component layout on either side ofan isolation galvanic barrier.
 14. The method of claim 1 wherein thefirst switch comprises a first transistor and the second switchcomprises a second transistor.
 15. A switching mode power convertercomprising: a. a transformer having a primary winding coupled to aninput supply voltage and a secondary winding; b. a first switch coupledin series to the primary winding; c. a state machine coupled to thefirst switch, wherein the state machine is configured to output a firstswitch driving signal to drive the first switch ON and OFF; d. a secondswitch coupled in series to the secondary winding; e. a controllercoupled to the second switch, wherein the second controller isconfigured to turn the second switch ON and OFF; f. a sensing circuitcoupled to the secondary winding and the controller, wherein the sensingcircuit is configured to sense an output characteristic of the powerconverter, wherein the controller is configured to generate a secondswitch driving signal and apply the second switch driving signal to thesecond switch, wherein the second switch driving signal is modulatedbased on the measured output characteristic, and applying the secondswitch driving signal to the second switch results in a first voltagepulse across the secondary winding, the first voltage pulse comprising apulse width that corresponds to a magnitude of an out of regulationoutput characteristic, further wherein the transformer is configured asa signal transmitter to transmit the first voltage pulse from thesecondary winding to a primary side of the transformer, wherein thestate machine is configured to input a second voltage pulse proportionalto the first voltage pulse across the secondary winding and to generatethe first switch driving signal modulated according to the pulse width.16. The power converter of claim 15 wherein the controller and thesensing circuit are configured to determine for each switching cycle ofthe power converter when a power delivery from a primary side of thetransformer to the output circuit is completed, and when the powerdelivery is completed then the controller is configured to generate thesecond switch driving signal.
 17. The power converter of claim 16wherein the power delivery is completed when a current through thesecondary winding drops to zero.
 18. The power converter of claim 16wherein the power delivery from the primary side to the output circuitcorresponds to the first switch being OFF, and the controller isconfigured to generate the second switch driving signal while the firstswitch is OFF.
 19. The power converter of claim 15 wherein the statemachine is a pulse width modulation circuit and the first switch drivingsignal comprises a pulse width modulated voltage signal.
 20. The powerconverter of claim 19 wherein pulse width modulation circuit isconfigured to modulate a duty cycle of the pulse width modulated voltagesignal output from the pulse width modulation circuit based on thereceived second voltage pulse.
 21. The power converter of claim 15wherein the output characteristic is one or more of an output voltage,an output current, and an output power of the power converter.
 22. Thepower converter of claim 15 further comprising an auxiliary windingcoupled to the state machine, wherein the auxiliary winding ismagnetically coupled to the secondary winding.
 23. The power converterof claim 22 wherein the transformer is configured to transmit the firstvoltage pulse from the secondary winding to the auxiliary winding usingthe magnetic coupling between the secondary winding and the auxiliarywinding.
 24. The power converter of claim 22 wherein the transformer isconfigured to transmit the first voltage pulse from the secondarywinding to the auxiliary winding using a parasitic capacitance between aprimary side and a secondary side of the power converter.
 25. The powerconverter of claim 24 wherein the parasitic capacitance comprises aparasitic capacitance of the transformer.
 26. The power converter ofclaim 24 wherein the parasitic capacitance comprises an inherentcapacitance of a printed circuit board onto which the power converter isassembled, wherein the inherent capacitance is a result of a componentlayout on either side of an isolation galvanic barrier.
 27. The powerconverter of claim 15 wherein the first switch comprises a firsttransistor and the second switch comprises a second transistor.
 28. Thepower converter of claim 15 wherein the sensing circuit comprises avoltage divider circuit.
 29. The power converter of claim 15 furthercomprising a diode coupled in parallel to the second switch and anoutput capacitor coupled in series to the diode, wherein the diode isconfigured to enable current flow from the secondary winding to theoutput capacitor when forward-biased.
 30. The power converter of claim29 wherein when the second switch is ON, an alternative current path isformed between the output capacitor and the secondary winding of thetransformer, further wherein a negative secondary current flows from theoutput capacitor to the secondary winding via the alternative currentpath, thereby forming the voltage pulse across the secondary winding.31. The power converter of claim 30 wherein the negative secondarycurrent is generated as discharge from the output capacitor when thealternative current path is formed.
 32. The power converter of claim 15wherein the power converter is configured as one of a flyback-type powerconverter circuit, a forward-type power converter circuit, apush-pull-type power converter circuit, a half-bridge-type powerconverter circuit, and a full-bridge-type power converter circuit.